The present invention relates to a liquid crystal display device, more especially to the improvement of a process for preparing an active matrix wiring substrate involved in it.
FIG. 3 is a schematic view of a channel-etched TFT formed on an active matrix wiring substrate involved in a conventional liquid crystal display device. FIG. 3(a) is a plan; FIG. 3(b), a cross sectional view; and FIG. 3(c), a fragmentary cross sectional view showing its terminal. Referring to FIG. 3(b), a gate electrode 2a is formed on a transparent insulating substrate 1. A gate insulating film 3 is formed to cover the gate electrode 2a. A semiconductive layer 4 is further formed thereon so as to overlie the gate electrode 2a. Source electrode 6a and drain electrode 7 are separated from each other on the central of the semiconductive layer 4, and connected to the same semiconductive layer 4 through an interposed ohmic is etched to contact layer 5. An area of the ohmic contact layer 5 disposed between the source and drain electrodes 6a and 7 leave it only between the semiconductive layer 4 and each of the source and drain electrodes 6a and 7. Further, a passivation film 17 is formed thereon so as to cover the surface thereof. On the passivation film 17, a transparent conductive film to provide a pixel electrode 9 is connected to the drain electrode 7 interposed with a contact through-hole 11 formed through the passivation film 17.
Next, a process for preparing the active matrix wiring substrate shown in FIG. 3 will be explained below in reference to FIG. 4.
(A) A first patterning step is carried out in which a conductive layer made of Al, Mo, Cr or the like is deposited on a transparent insulating substrate made of glass or the like to a thickness of 100 to 200 nm with a sputtering apparatus, and then gate wire 2b, gate electrode 2a and gate terminal portion 2c which is to be connected to an outside signal processing substrate for displaying are formed by a photolithographic step.
(B) Next, a second patterning step is carried out in which a gate insulating film 3 made of silicon nitride or the like, a semiconductive layer 4 made of amorphous silicon and an ohmic contact layer 5 made of n+-type amorphous silicon are laminated successively in this order to a thickness of about 400 nm, about 300 nm and about 50 nm, respectively with a PCVD apparatus, and then the semiconductive layer 4 and ohmic contact layer 5 are patterned at the same time.
(C) Then, a third patterning step is carried out in which source electrode 6a, source wire 6b, drain electrode 7 and data side terminal portion 7a are formed by photolithographic processing after depositing Mo, Cr or the like to a thickness of about 150 nm with a sputtering apparatus so as to cover the gate insulating film 3 and ohmic contact layer 5. Unnecessary part of the ohmic contact layer 5 is removed which is the part except a portion that is positioned under the source and drain electrodes 6a and 7 to form a channel part of a TFT.
(D) Thereafter, a forth patterning step is carried out in which an inorganic passivation film 17 of silicon nitride is formed to a thickness of about 100 to 200 nm with a PCVD apparatus so as to cover a back channel of the TFT, source electrode 6a, source wire 6b, drain electrode 7 and the terminal portions; a contact through-hole 11 is formed for bringing the drain electrode 7 into contact with a pixel electrode 9; and unnecessary part of the passivation film 17 which is located on the data side terminal 7a portion and unnecessary parts of the gate insulating film 3 and the passivation film 17 which are located on the gate terminal 2c portion are removed.
(E) Finally, a fifth pattering step is carried out after forming a transparent conductive film which is to be changed into the pixel electrode 9 with a sputtering apparatus.
By the above explained five patterning steps, a liquid crystal display device having the active matrix wiring substrate shown in FIG. 3 whose preparing steps are greatly reduced can be prepared.
However, the above conventional liquid crystal display device (hereinafter referred as to xe2x80x9cprior art Ixe2x80x9d) requires screening of light by means of a black matrix provided on a CF substrate in order to inhibit the leak of light from the space between the gate wire 2b and the pixel electrode 9 and from the space between the source wire 6b and the pixel electrode 9 as shown in FIG. 3(a). In order to avoid the problems concerning the accuracy in superimposing the CF substrate on the active matrix wiring substrate, the light screening region of the black matrix needs to have a large space. As a result, the aperture ratio of the liquid crystal display device becomes low. On this account, the prior art I has such a problem that the transmittance of the liquid crystal display device becomes low.
Japanese Patent Kokai-Publication JP-A-9-152625(1997) (hereinafter referred as to xe2x80x9cprior art IIxe2x80x9d) discloses, as a means of increasing the aperture ratio, a process for overlapping the pixel electrode 9 with each of the wires and thereby removing the black matrix of the CF side. FIG. 5 is a cross sectional view showing a channel protecting TFT on an active matrix wiring substrate of the prior art 2. Referring to FIG. 5, the structure of the channel protecting TFT in the active matrix wiring substrate will be explained as follows. There are a transparent insulating substrate 1 and a gate electrode 2a which is provided thereon and connected to a gate wire 2b. They are covered with a gate insulating film 3 on which a semiconductive layer 4 is provided so as to overlie the gate electrode 2a. On the central part of the semiconductive layer 4, a channel protecting layer 13 is provided. There is provided a n+-type Si layer which covers both terminals of the channel protecting layer 13 as well as a part of the semiconductive layer 4 and is divided into two pieces to provide source electrode 6a and drain electrode 7. On the outside terminal of one piece of the n+-type Si layer which is to be the source electrode 6a, transparent conductive film 14 and metallic later 15 are provided in this order to form a source wire 6b of a two-layered structure. Similarly, on the outside terminal, other piece of the n+-type Si layer which is to be drain electrode 7, transparent conductive film 14 and metallic later 15 are provided in this order. The transparent conductive film 14 is prolonged and connected to the pixel electrode 9 to form a connecting electrode. Further, there is provided an interlaminar insulating film which covers the TFT, the gate wire 2b, the source wire 6b and the connecting electrode. On the interlaminar insulating electrode, a transparent conductive film which is to be the pixel electrode 9 is provided and connected to the drain electrode 7 of the TFT by the transparent conductive film of the connecting electrode through a contact through-hole formed through the interlaminar insulating film.
These characteristic features of the liquid crystal display device of the prior art II reside in that the pixel can overlap the wires without increasing a capacity between the pixel electrode 9 and each of the wires to provide a liquid crystal display device having a large aperture ratio and thereby being capable of displaying a bright image by forming a low dielectric interlaminar insulating film thickly between the pixel electrode 9 and the source electrode 6a as well as between the pixel electrode 9 and source wire 6b. 
However, there have been encountered various problems during the course of the investigations toward the present invention. Namely, the aforementioned prior art II requires 9 patterning steps including newly additional preparing steps for forming the interlaminar insulating film and for making the contact through-hole which is to be in contact with the pixel electrode. Accordingly, a problem is caused that the production cost of the liquid crystal display device needs must be increased greatly.
Further, where it is attempted to apply the art of improving the aperture ratio by using this interlaminar insulating film to the process for preparing the active matrix wiring substrate of the prior art I having a reduced number of manufacturing steps, it is necessary to carry out dry etching to remove unnecessary part of the passivation film 17 located on the data side terminal 7a portion, which simultaneously removes unnecessary parts of the gate insulating film 3 and passivation film 17 located on the gate terminal 2c portion by applying a photosensitive organic interlaminar film 8c as a mask as shown in FIGS. 6(a) and 6(b). In case where the gate insulating film 3 and the passivation film 17 which are made of silicon nitride are etched, fluorine-containing gas is used. This causes a problem that the photosensitive organic interlaminar film 8c is also etched off.
Alternatively, another process could be also considered for improving the aperture ratio which is similar to the above process but different in using a non-photosensitive acrylic resin, coating a positive photosensitive resist (photoresist) thereon, and etching the acrylic resin and the silicon nitride film at the same time. However, obtainable etching selectivity of the positive photoresist to the silicon nitride film is no more than about 1. Accordingly, even necessary part of the positive photoresist would disappear in the course of etching the acrylic resin and the silicon nitride film. This causes a problem of also etching necessary part of the acrylic resin which cannot be avoided. Further, there is also caused another problem of increase in the cost as compared with the channel-etched TFT of the prior art 1 by forming the interlaminar insulating film consisting of the organic insulating film on the passivation film 17 made of silicon nitride film deposited by using a conventional PCVD apparatus.
In order to solve these problems, it has been studied to impart the functions of not only the interlaminar insulating film but also the passivation film 17 to the organic insulating film without forming the expensive passivation film which requires the use of the PCVD apparatus. However, the passivation film requires the function of blocking impurity ions and moisture which penetrate into the channel portion in order to maintain the reliability of the liquid crystal display device. When the organic insulating film made of acrylic resin, polyimide resin or the like is directly in contact with the channel portion, a problem is entrained that transistor properties deteriorate by the penetration of impurity ions and moisture originated from a liquid crystal material and/or of ions originated from the organic resins.
The present invention has been achieved in consideration of the above problems. It is an object of the present invention to provide a novel simple process for forming interlaminar insulating films between the pixel electrode and wires in the liquid crystal display device, which includes an active matrix wiring substrate having a structure of overlapping the wires with the pixel electrode.
Also it is another object of the present invention to provide an active matrix type liquid crystal display device with high transmittance and capable of bright displaying as well as a process for manufacturing same, which can be prepared at low cost only through 5 photolithographic steps, i.e., without increasing the photolithographic steps as compared with those involved in a process for preparing the conventional liquid crystal display device having high aperture ratio.
According to an aspect of the present invention there is provided a liquid crystal display device which comprises first and second organic interlaminar insulating films between pixel electrode(s) and wire(s). The first organic interlaminar insulating film directly covers source electrode(s), source wire(s), drain electrode(s) and back channel(s). Underlying one of the organic interlaminar insulating films is directly in contact with a channel part of a TFT.
In the above liquid crystal display device of the present invention, the first organic interlaminar insulating film preferably contains at least one organic layer forming material selected from the group consisting of polysilazane, siloxane resin and benzocyclopolybutene polymer.
Preferable water absorption of the organic layer forming material is not more than 1%.
The second organic interlaminar insulating film preferably contains an organic layer forming material of an acrylic family resin which is soluble in dimethylene glycol methylethyl ether.
According to a second aspect of the present invention, there is provided a process for preparing a liquid crystal display device, including the steps of:
(A) forming a metallic thin film on a transparent electrode substrate(preferably with a sputtering apparatus) followed by forming gate electrode, gate wire and gate terminals by photolithographic processing;
(B) forming successively a-Si layer which forms a semiconductive layer and n+-type Si layer which forms an ohmic contact layer to cover the gate electrode and the gate wire followed by pattering both of the layers into islands;
(C) forming a metallic thin film on the both layers (preferably with a sputtering apparatus) followed by forming source electrode, source wire, drain electrode and data side terminal by photolithographic processing, removing unnecessary part of the n+-type Si layer between the source electrode and the drain electrode to form a back channel;
(D) forming a first organic interlaminar insulating layer over the laminated surface of the substrate including the back channel (e.g., by spin coating) followed by curing completely, forming a second organic interlaminar insulating layer thereon (e.g., by spin coating) followed by half curing, coating further thereon a positive photoresist containing a novolak resin as its essential ingredient (e.g., by spin coating) followed by pre-baking, partially light-exposing the positive photoresist with a light-exposing apparatus to be selectively solbilized and at the same time selectively solvilizing the second organic interlaminar insulating film and removing the solvilized portion of the positive photoresist and the second organic interlaminar insulating film therebeneath to make a contact-through hole;
(E) after mid-baking in an oven, removing unnecessary parts of the first organic interlaminar insulating film and the gate insulating film which are uncovered with a mask of the positive photoresist remained followed by removing the positive photoresist; and
(F) forming a transparent conductive film (preferably by using a sputtering apparatus) followed by patterning it into pixel electrode(s).
In the above process of the present invention, the first organic interlaminar insulating film found in the step (D) preferably contains at least one organic layer forming material selected from the group consisting of polysilazane, siloxane resin and benzocyclopolybutene polymer.
Preferable water absorption of the organic layer forming material is not more than 1%.
The second organic interlaminar insulating film preferably contains an organic layer forming material of an acrylic family resin which is soluble in dimethylene glycol methylethyl ether.
Preferable solvent of the novolak resin found in the (D) step is a mixture of 2-heptane and ethyl 3-ethoxydiazidosulfonate.
It is preferable that the half curing of an acrylic family resin in the step (D) is performed by pre-baking at 100 to 200xc2x0 C. for 1 to 4 minutes and that the novolak resin is pre-baked at temperatures ranging from 90 to 120xc2x0 C. which do not exceed the pre-baking temperature of the second organic interlaminar insulating film for 1 to 4 minutes.
The positive photoresist in the step (D) is preferably developed by using a tetramethyleneammonium hydroxide solution.